QGis raster layer as background layer

[TNick]

I have a QGis project saved in the PostgreSql with three layers saved in the same database (two vector layers and a raster layer). As it is the webapp shows the three layers inside the theme and renders the map without issue.

I would like the raster layer to be a background layer (not show up in the list of theme layers, show up as a background layer).

From other answers elsewhere I take the hint that I should use a relative path but can't seem to find a good example for that. The same issue seems to suggest that I should create a separate theme (project) for the raster layer, maybe.

[manisandro]

You can define a backgroundLayer with just the one layername from your QGIS project and then define the same layer as a printLayer. This way, it will not appear in the QWC2 layer tree, will appear in the background layer selection, and will be used as a background when printing.

Something along the lines of this very minimal example should work:

{
  "themes": {
    "items": [
      {
        "url": "/ows/my_project",
        "backgroundLayers": [
          {"name":"my_bg_layer","printLayer":"qgis_layer_name"}
        ],
        "mapCrs": "EPSG:XXXX"
      }
    ],
    "backgroundLayers": [
      ,
      {
        "type":"wms",
        "url":"/ows/my_project",
        "name":"my_bg_layer",
        "srs":"EPSG:XXXX",
        "params":{"LAYERS":"qgis_layer_name"}
      }
    ]
  }
}

[TNick]

That works beautifully, thank you.

What if I want to use wmts? When adding it like a regular raster layer all I had to do was to add "tiled": true, inside theme definition, so I assume some behind-the-scene magic happens. However, simply changing the type in the background layer definition was not enough. I assume I have to manually provide tileMatrixPrefix, tileMatrixSet, originX, originY, resolutions and tileSize?

[TNick]

I did some more digging:

• in the host machine I copied the wmts_config_generator.py utility from QWC2 project into volumes/qgs-resources/scripts/wmts_config_generator.py
• for some reason I cannot enable port mapping so that port 80 from qgis server si available on host machine
• docker exec -it qwc-qgis-server-1 sh to get into qgis server prompt
• python3 data/scripts/wmts_config_generator.py "http://127.0.0.1/ows/pg/public/MAPNAME?SERVICE=WMTS&REQUEST=GetCapabilities" LAYER_NAME "EPSG:XXXX"
• the layer is not found
• I modified wmts_config_generator.py so that it prints the layer it found; if finds no layer
• in the QGis project that hosts the layer you need to make sure that WMTS is enabled for that layer (project properties -> QGis Server -> WMTS tab)
• now the wmts_config_generator.py utility generates a configuration which I merged with the previous hint, resulting:

            {
                "url": "/ows/pg/public/2020-10-07-background",
                "srs": "EPSG:3844",
                "type": "wmts",
                "params":{
                    "LAYERS": "ortofotoplan_2021"
                },
                "title": "Ortofotoplan 2021",
                "name": "ortofotoplan_2021",
                "format": "image/png",
                "requestEncoding": "KVP",
                "tileMatrixPrefix": "",
                "tileMatrixSet": "EPSG:3844",
                "originX": 1396130.211,
                "originY": -334742.867,
                "projection": "EPSG:3844",
                "tileSize": [
                  256,
                  256
                ],
                "style": "default",
                "bbox": {
                  "crs": "EPSG:4326",
                  "bounds": [
                    25.523214,
                    46.557917,
                    25.825315,
                    46.884994
                  ]
                },
                "resolutions": [
                  6963.833519999999,
                  3481.9167599999996,
                  1740.9583799999998,
                  870.4791899999999,
                  435.23959499999995,
                  217.61979749999998,
                  108.80989874999999,
                  54.404949374999994,
                  27.20247468764,
                  13.601237343679998,
                  6.800618671839999,
                  3.4003093359199994,
                  1.7001546679599997,
                  0.85007733412,
                  0.42503866692,
                  0.21251933359999997,
                  0.10625966679999999,
                  0.05312983339999999,
                  0.02656491656
                ],
                "thumbnail": "ortofotoplan_2021.jpg"
            },

However, something is off, as the background tiles are empty. I see requests like these, return code is 200 but there's no content.

https://voslabeni.advaita.ro/ows/pg/public/2020-10-07-background
  ?layer=ortofotoplan_2021
  &style=default
  &tilematrixset=EPSG%3A3844
  &Service=WMTS
  &Request=GetTile
  &Version=1.0.0
  &Format=image%2Fpng
  &TileMatrix=16
  &TileCol=-31242
  &TileRow=-33306

[manisandro]

Note that tiled: true just enables tiling mode in WMS, it is not a WMTS. If you want to enable tiling mode in your background layer, you can add TILED=1 to the params. If you want to use WMTS proper, you'll need to set up a tile cache.

[TNick]

Thank you for your reply.

Setting TILED inside params to 1 or true seems to have no effect. The request is for the full width and TILED parameter is false.

            {
                "url": "/ows/pg/public/project-name",
                "srs": "EPSG:3844",
                "type": "wms",
                "params":{
                    "LAYERS": "ortofotoplan_2021",
                    "TILED": true
                },
                "title": "Ortofotoplan 2021",
                "name": "ortofotoplan_2021"
            }

Setting it outside to true (not 1) causes the request to have the TIELD parameter true but there is a single request for the whole background (not tiled). Setting tileSize (borrowed from other background layer def) in params and outside did not help.

            {
                "url": "/ows/pg/public/project-name",
                "srs": "EPSG:3844",
                "type": "wms",
                "params":{
                    "LAYERS": "ortofotoplan_2021",
                    "tileSize": [
                        256,
                        256
                    ]
                },
                "title": "Ortofotoplan 2021",
                "name": "ortofotoplan_2021",
                "tiled": true,
                "tileSize": [
                    256,
                    256
                ]
            },

---

Regarding your second point, I would expect the tile cache to be transparent (in the sense that the request would look the same, the web app would be oblivious about the existence of a cache between it and the server), right? While inefficient, the QGis server should reply with png tiles. What I get is an empty response.